I've built an "evil black rifle" from collected parts. Bushmaster complete lower and Model 1 bolt assembly but I bought the upper from my dad's surplus of AR uppers. He's had them for quite some time and they were all in good shape, if a little dusty. Everything fits snug and goes bang every time I want it to. My question is how can I determine the manufacture of the upper? Just for the sake of ending my curiosity on the matter.